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How Sydney helped uncover Angkor's hidden history

Using a radical new imaging technique, Sydney archaeologists have uncovered a network of unknown temples and an entire city beneath the foliage surrounding Angkor in Cambodia. [Image: Michael Coe]

For more than one and a half centuries, explorers and scientists alike have relied on the machete to clear the dense vegetation that obscures much of the remains of the great medieval civilisation of Angkor, which flourished across mainland Southeast Asia from the 9th to 15th centuries AD. Until the past year, that is.

In July 2012 Roland Fletcher, a Professor in the Department of Archaeology, and I sat in an airconditioned office at the University's Robert Christie Research Centre in Cambodia, having just taken delivery of the results of a landmark aerial survey, which deployed an airborne laser scanner strapped to the side of a helicopter. The whole project had been something of a gamble: several years of planning and a quarter of a million dollars invested in a technology.

The laser scanner, or 'lidar', had the potential to see through a dense forest canopy and reveal traces of the civilisation remaining on the forest floor, but no-one was entirely sure it would work. As a map derived from four billion laser measurements of Angkor slowly unfolded on the screen of a high-performance computer, however, we watched in awe as entire cities were revealed for the first time beneath the jungle of northwest Cambodia, and realised that the age of the machete had just drawn to a decisive close.

The great monuments of Angkor, along with their inscriptions and artworks, have long fascinated scholars, and over the past century a huge body of work has been produced on these aspects of Khmer civilisation. In the 1990s, as decades of conflict in Cambodia finally wound to an end, the arrival of modern archaeological methods sparked a renewed interest in Angkor, but with an entirely different focus. For the first time, scholars began seriously to address broader questions about the context of the temples: Who exactly were the people who built them, where did they live, how were they so successful in the unforgiving environment of monsoon Asia, and perhaps most importantly, what happened to them?

The problem for archaeologists had always been that houses of stone were reserved for the gods, and that the stuff of everyday life was mostly non-durable material - houses made of thatch, and even royal palaces made of wood - that has long since disappeared. In the 1992 however, a French archaeologist, Professor Christophe Pottier, noted that traces of the remains of ponds, occupation mounds, village shrines, roadways and canals could still be discerned from the air. It was the fabric of the urban and agricultural network of greater Angkor.

Pottier began to use aerial photos to sketch those traces by hand onto paper maps from the 1950s, setting in motion a process that would lead eventually to the lidar survey two decades later. A meeting between Pottier and Fletcher in 1998 laid the groundwork for the University of Sydney's involvement in Angkor. The technical skills of the Archaeological Computing Laboratory (now Arts eResearch) were brought to bear on Pottier's maps, which were converted into an enormous digital mapping database and, as Pottier jokes, "allowed me to enter honourably into the 21st century".

For the next decade and a half, Professors Fletcher, Pottier and I continued the mapping work, eventually expanding the inventory of temples in the Greater Angkor area from around 350 to 1250, and discovering that Angkor was actually the largest integrated settlement complex of the pre-industrial world, comparable in size to modern-day Sydney.

The discovery was remarkable, but there were no 'eureka moments', just a gradual accumulation of more and more pieces of the puzzle over the course of many years of painstaking analysis and difficult fieldwork. Most troubling of all was the fact that vast swathes of the new archaeological map consisted of nothing more than white space, where forest cover prevented conventional technologies like satellites and airborne radar from identifying any traces of the civilisation that might remain etched into the surface of the landscape.

In 2005, Roland and I began to hatch a plan to use lidar's capacity to 'see through' the forest to fill in those blank spaces on the map. It was an untested technology and many researchers working in Cambodia were sceptical that the instrument could see through the dense vegetation and deliver worthwhile results. No-one was prepared to commit the six-figure sum the mission required. Encouraged by a successful application of the technology in a similar context at a Mayan site in Belize, however, and with seed money from National Geographic, we spent years essentially going door-to-door seeking the participation of the various other international teams working at Angkor.

Convincing the Cambodian authorities of the merits of the idea was a different matter entirely: the permissions process took six months, involved several different ministries, required an unprecedented exemption from the no-fly zone above Angkor Wat, and went all the way up to prime ministerial level. In the end, eight different teams representing seven different countries committed support, in what would turn out to be the broadest research cooperation ever achieved in Cambodian archaeology, and the largest archaeological lidar acquisition ever undertaken anywhere in the world.

In April 2012 the Indonesian branch of a Canadian survey company, PT McElhanney, was contracted to import the necessary equipment and undertake the lidar survey. A red helicopter spent nearly a week systematically criss-crossing the airspace just 800 metres over the World Heritage Site of Angkor, as well as two other remote and forested locations.

The resulting imagery reveals that archaeological features are almost ubiquitous beneath the forest cover, and that the ceremonial centre of Angkor, with its great temples shrouded in jungle, was surrounded by a formally planned, grid-like network of roads. A French team had spent many years surveying one part of that 'downtown' area on the ground, using machetes and hand-operated survey levels; the lidar mission covered that same area in 45 minutes of flying time, and produced a map with greater precision and accuracy.

At least half a dozen previously undocumented temples have been uncovered in the immediate vicinity of Angkor Wat, which more than two million tourists visit every year, along with a previously unknown urban layout within the very confines of the temple's moat.

In the remote Kulen mountains to the north of Angkor, where dense forest and extensive mine fields have traditionally frustrated mapping efforts, an entire urban layout has emerged from beneath the vegetation. It corresponds to a previously undiscovered city referred to in thousand-year-old inscriptions as Mahendraparvata.

The newly discovered cities clearly extend beyond the limited lidar coverage that has so far been achieved, and we are currently fundraising for a second lidar mission to take a first look at a couple of other temple complexes in the region where, we suspect, entire cities also lie undiscovered on the forest floor.

Our job will be significantly easier this time around: thanks to the first results, the initial scepticism surrounding the method has largely evaporated, and has now given way to enthusiasm. The results, according to Professor Fletcher, are "a total gamechanger" for archaeology in the region.